This invention relates generally to mercury removal and, more particularly, to the removal of mercury from gaseous process streams.
Mercury is recognized as a toxic material, the presence of which can cause or result in significant health hazards. Further, the presence of mercury is known to create, cause or result in a danger of corrosion of certain materials which come into contact therewith.
Natural gas, depending on its origin, may contain variable quantities of mercury. It is known that mercury in natural gas can cause failures in associated process equipment such as in aluminum heat exchangers.
A commonly employed method for removing mercury from natural gas streams is based on the passage of the mercury-containing stream through a non-regenerable sulfur-impregnated carbon bed. In practice, the beds employed in such treatment schemes are necessarily large in order to ensure against mercury breakthrough while avoiding frequent changeouts of absorbent. As will be appreciated, reducing or minimizing the frequency of absorbent changeouts is generally desired as the associated facility may not operate during the changeout operation. Also, changeout operations are typically very labor intensive and may also pose significant health or other hazards to the involved personnel. Further, disposal of the spent material and recovery of the mercury engenders additional costs and scheduled downtime.
In general, the use of non-regenerable sorbent beds does not admit of an easy way by which the degree to which a sorbent has been used or spent can be sampled. Consequently, an operator is generally left with no choice but to change all of the sorbent when it is convenient to do so whether it is necessary or not. Further, an unexpected or prolonged period with abnormally high amounts of mercury in the gas to be treated may cause an overload on the absorbent such as to result in bed breakthrough with consequent damage to the plant or an alarm condition such as may necessitate a plant shutdown.
In addition to natural gas in a gaseous or liquid state, other fluids such as various flue gas process streams are also known to include or contain significant quantities of mercury such that the removal of mercury therefrom may be desired or required. For example, environmental standards for particulate and total mercury emissions from coal-fired power plants, petroleum refineries, incinerators, metallurgical operations, thermal treatment units and other particulate- and mercury-emitting facilities are increasingly becoming more demanding. The U.S. Environmental Protection Agency (EPA) has indicated that over 50 tons of mercury are emitted annually as a result of coal utilization in the utility industry. To that end, the EPA is looking to determine the appropriateness and need to regulate toxic air pollutant emissions (focusing on mercury) from coal-fired utility boilers.
Various flue gas emission control strategies or schemes have been developed. Common power plant operations strategies have, for example, included or employed coal cleaning, electrostatic precipitators, fabric filters, scrubbers, spray dryers, and carbon adsorption. While existing flue gas emission control strategies or schemes are generally capable of significantly reducing the potential amount of mercury released to the atmosphere, the effectiveness of such controls can vary considerably on either a plant-by-plant or a boiler-by-boiler basis.
Any contemplated regulation will likely be directly dependent on the availability of cost-effective control technologies capable of implementation in a very diverse coal-fired utility industry.
In view of the above, there is a continuing need and demand for efficient, low-cost technologies for mercury removal from gaseous streams. In particular, there is a need and a demand for efficient, low-cost technologies for removing mercury from natural gas streams and for mercury emission reductions in flue gas streams from utility plants such as employ coal combustion.
Wilhelm, U.S. Pat. No. 5,238,488, issued Aug. 24, 1993 and assigned to Gas Research Institute discloses a process and solution for transforming insoluble mercury metal into a soluble compound such as may be applied for cleaning equipment already contaminated with mercury. The process includes oxidizing elemental mercury to form mercury cations, contacting the mercury cations with a complexing agent to form a soluble complex, and dissolving the soluble complex with a solvent. The solution contains an oxidizing agent (e.g., nitric acid), a complexing agent and a solvent.
However, a need and demand for efficient, low-cost technologies for mercury removal from gaseous streams remains.
A general object of the invention is to provide an improved technique for the removal of mercury from gaseous process streams.
A more specific objective of the invention is to overcome one or more of the problems described above.
The general object of the invention can be attained, at least in part, through a process wherein a first mercury-containing gaseous process stream is contacted with a regenerable mercury scavenger solution to form a treated stream having a reduced mercury content as compared to the first mercury-containing gaseous process stream and a used scavenger solution having an increased mercury content. In accordance with one preferred embodiment of the invention, the regenerable mercury scavenger solution comprises a solution which contains:
a) nitric acid effective to oxidize mercury contained within the first mercury-containing gaseous process stream to form mercury cations,
b) a complexing agent suitable for forming a soluble mercury compound when contacting the mercury cations, and
c) a solvent effective to dissolve the soluble mercury compound.
The prior art generally fails to provide a desirably low-cost process for the removal of mercury from gaseous process streams such as streams which are formed by or include natural gas or flue gas streams such as resulting from coal combustion. In particular, the prior art generally fails to provide an approach for the removal of mercury from such gaseous streams which permits as easy as desired measurement or determination of remaining mercury absorption capacity. Still further, the prior art generally fails to provide for regeneration of mercury removal capacity on a continuous or semi-continuous basis such as may be desired for more efficient and/or effective operation or processing.
The invention further comprehends a process for removing mercury from a flue gas process stream. In accordance with one preferred embodiment of such process, a mercury-containing flue gas process stream is contacted with a regenerable mercury scavenger solution to form a treated stream having a reduced mercury content as compared to the mercury-containing flue gas process stream and a used scavenger solution having an increased mercury content. The used scavenger solution is then regenerated. The regenerable mercury scavenger solution employed in such processing preferably comprises a solution containing:
a) nitric acid effective to oxidize mercury contained within the mercury-containing flue gas process stream to form mercury cations,
b) a complexing agent suitable for forming a soluble mercury compound when contacting the mercury cations, and
c) a solvent effective to dissolve the soluble mercury compound.
The invention still further comprehends a process for removing mercury from a natural gas stream. In accordance with one preferred embodiment of such process, a mercury-containing natural gas feed stream is contacted with a regenerable mercury scavenger solution to form a treated stream having a reduced mercury content as compared to the natural gas feed stream and a used scavenger solution having an increased mercury content. The used scavenger solution is then regenerated. The regenerable mercury scavenger solution employed in such processing preferably comprises a solution containing:
a) nitric acid effective to oxidize mercury contained within the natural gas feed stream to form mercury cations,
b) a complexing agent suitable for forming a soluble mercury compound when contacting the mercury cations, and
c) a solvent effective to dissolve the soluble mercury compound.
Other objects and advantages will be apparent to those skilled in the art from the following detailed description taken in conjunction with the appended claims and drawing.